late 14c., megrim, from Old French migraigne (13c.), from vulgar pronunciation of Late Latin hemicrania "pain in one side of the head, headache," from Greek hemikrania, from hemi- "half" + kranion "skull" (see cranium). The Middle English form was re-spelled 1777 on the French model. Related: Migrainous.

migraine mi·graine (mī'grān')n. A severe recurring headache, usually affecting only one side of the head, that is characterized by sharp pain and is often accompanied by nausea, vomiting, and visual disturbances. Also called hemicrania, megrim, sick headache.

(mī'grān') A severe recurring headache, usually affecting only one side of the head, that is characterized by sharp, throbbing pain and is often accompanied by nausea, vomiting, sensitivity to light, and visual disturbances. Vasodilation in the brain causes inflammation that results in pain, but the exact cause of migraine is unknown.